1. Field of the Invention
This invention relates to a stop-down signal adjusting circuit used for a camera with an automatic aperture control mechanism.
1. Background of the Invention
A so-called "full-aperture metering system" in which light is received inside a camera to make measurements of light is extensively employed for cameras. More specifically, in such a system, a shutter speed, film sensitivity and exposure value are preset, and then a suitable aperture value is calculated from the quantity of light passed through a fully opened photographing lens. After the shutter is released, the lens is mechanically stopped down to the aperture value thus calculated.
In the metering system, it is essential that the aperture value is detected to provide a stop-down signal during the stopping down of the lens. The stop-down signal is fed back to the calculation circuit of the camera to be compared with the calculated aperture value, and the lens stopping operation is suspended upon coincidence of the two.
Means are available for detecting the lens stopping operation to produce the stop-down signal. One example is a device in which an oscillation wave signal produced in association with the lens stopping operation is converted into pulses, and the stop-down signal is outputted according to the count value of the pulses.
FIGS. 5 and 6 show one example of a conventional device for producing an oscillation wave signal in association with a lens stopping operation. In FIGS. 5 and 6, reference numeral 1 designates a lever operated in association with the operation of a lens stopping member. The lever 1 is moved in the direction of the arrow as the lens is stopped down. Since a pin 1a embedded in the lever is engaged with a fork-shaped portion 2a of a sector-shaped gear 2, the movement of the lever 1 turns the gear 2, which rotates an intermediate gear 3. The rotation of the gear 3 is transmitted through a gear 4 to a link gear 5 to rotate the link gear 5.
A code board 6 having a number of radially extending slits in its periphery is coaxially mounted on the gear 5. The code board 6, together with a light emitting diode 7 and a photo transistor 8 arranged on either side of the periphery of the code board 6, form a photo chopper.
In the conventional device thus constructed, the code board 6 is turned as the lens is stopped down, so that the photo chopper outputs an oscillation wave signal having a number of cycles corresponding to the current aperture value. As shown in a block diagram of FIG. 7, the oscillation wave signal from the photo chopper 100 is converted into a pulse signal by a pulse converter 102, and the pulses are counted by a counter 104. The count value of the counter 104 comprises a stop-down signal and is compared in comparator 106 with the calculated aperture value. When the stop-down signal coincides with the aperture value, an electromagnetic drive 9 unit is operated to suspend the stop-down operation. As shown in FIG. 5, the electromagnetic drive unit 9, when activated, turns a stop lever 11 about a pin 10 until the locking pawl of the stop lever 11 is engaged with the gear 4. As a result, the rotation of the gear 4 is stopped, and the lever 1 is also stopped simultaneously, so that the lens stopping operation is stopped in agreement with the control value.
In the above-described device, a peak value of the oscillation wave signal depends very much on the accuracy and quality of the components of the photo chopper, such as the photo transistor. This cannot be disregarded in the conversion of the oscillation wave signal into the pulse signal.
FIG. 8(a) shows oscillation wave signals V.sub.0, V.sub.1 and V.sub.2 provided respectively when three different photo transistors are used. FIG. 8(b) shows stop-down signals, i.e., pulse P.sub.1 and P.sub.2, which are obtained through pulse conversion by the pulse converter 102 having a reference voltage V.sub.ref.
As is apparent from FIGS. 8(a) and (b) in the case of the low amplitude oscillation wave signal V.sub.0, the pulse conversion cannot be carried out, and the automatic aperture control cannot be performed. It is thus clear that, irrespective of the peak value of the oscillation wave signal outputted by the aperture value detecting source such as the photo chopper, the input voltage to the pulse converter must have a substantially constant peak value.